When it comes to solar panels, bigger isn’t always better. In fact, for many situations, a small solar panel is more than enough to charge a battery for backup power, caravans and RVs. But how big is big enough? In this post, we answer the question ‘what size solar panel do i need to charge a 100Ah battery’ – a common battery for mobile backup power or caravans and camping. We’ll break down the basics of calculating your solar needs and provide some tips for choosing the right sized solar panel to save money and never run out of power.

What is a 100Ah battery?

100 Amp Hours is a measure of how much electrical current your battery can provide. You can use this figure to help determine what size solar panels will charge the battery to full.

To work out solar power requirements, the other important thing you need to know about your battery is the battery voltage.

The voltage and the amp hours of a battery will tell you the battery capacity (in watt hours). Once you know this, you’ll know what you can power with the battery. And what size solar panels you need to charge it.

It’s also good know that 100Ah batteries commonly come in 12, 24 or 48 volts.

12v batteries are popular in caravans, RVs and for camping. They can also be used for backup power to run certain appliances at home, like a water pump in a power outage.

One thing know about 100Ah batteries is that the voltage of the battery should match the voltage of the solar panels and appliances you want to run.

So for 12V appliances, you should get a 12V, 100Ah battery.

You can find the voltage rating of the battery on the nameplate.

Different 100Ah battery types

100Ah batteries commonly come in two main types:

1. Lithium battery
2. Lead acid battery

Lithium batteries have a larger total battery capacity for their size. They typically have a longer battery life than lead acid batteries. They are also more expensive.

A lithium battery is a deep cycle battery. You can discharge it to a lower point than lead acid batteries. You can also charge it more quickly than you can charge a lead acid battery. You might discharge a lithium battery to 80% for example. A lead acid battery is more likely to discharge to 50% for most efficient use.

What is the size of a solar panel?

The size of solar panels (in terms of how much energy they output) is measured in watts.

12 volt solar panels come in a variety of sizes. But commonly, standard solar panels are sold in multiples of 10W up to 100 watts. The larger size solar panels come in 200 watts to 400 watts.

Large 12 volt solar panels are 160 to 200 watts.

It’s fairly common to get 100W portable panels, which can be chained together to increase the energy output as needed.

And then there’s the physical size of solar panels, which matter if you don’t have much space.

Solar panel dimensions vary with manufacturer and model.

Here are some typical dimensions for commonly used sizes of solar panels:

If you want to know more about portable solar panels, read our article 6 solar panel uses to secure your power supply.

To pair portable solar panels with a battery you also need a solar charge controller. This regulates the power between the solar panels and the battery.

What size solar panel do I need to charge a 100Ah battery?

The solar panel size you need to charge a 100Ah battery will depend on:

• how much power you want to generate,
• how quickly you want to charge your 100Ah battery, and
• how much physical space you have to work with.

It’s best to be able to charge your battery from the sun’s energy within a 5 hour period. You can only use the solar panels during daylight hours. And in Australia we get around 5 ‘peak sun hours’ in a day.

Charging your battery in 5 hours also means you can use a full battery overnight.

If you’re looking to charge a 100Ah battery rated at 12 volts in 1 day, then a single solar panel of 100 watts won’t be enough. We will explain why below.

You would use at least 3 x 100W solar panels to do this job.

If you want to factor in some rainy days where you can’t recharge your battery, it’s best to oversize your battery and solar panels.

The best way to figure out what size solar panel you need to charge a 100Ah battery is to work through some examples. We do this below.

How much power does a 100Ah, 12 volt battery need?

In this example we are going to assume that you have 5 hours of good sunlight to charge your 100Ah battery with solar panels. So what size solar panels do you need?

To calculate solar panel size, you need to first convert the Amp Hours of the battery into watts.

Converting Amp Hours to Watt hours

Solar systems come in watts. Converting your battery power from Amps Hours to Watts lets you compare apples with apples. This is where the battery voltage is important.

Converting amp hours is easy. The formula to convert Amps to Watts is:

Watts = (Amps x Volts)*Power Factor.

All of our examples assume a power factor of ‘1’ to make calculation easy.

What size solar panel for a 100AH battery?

A 100 amp hour 12V battery will produce 100 amperes of stored energy over one hour (100 amp hours).

Coverted to watts, that’s the same as 1200 watts in one hour (1200 watt hours). This is also how much power it takes to charge the battery from empty to full.

What size solar system will charge 100Ah battery (in 5 hours)?

When the battery is discharged, you’ll need to charge it with your solar system during daylight hours.

We’re going to assume you are using 12V solar panels, the same voltage as the battery.

In reality, an MPPT solar controller gives you the flexibility of using 24v or larger solar panels. It can also be more economically efficient to use higher voltage panels. But that is a topic for another post!

Here’s one way to work out what size solar panels are right for you:

In Australia, we use ratings from the Clean Energy Council to get the average daily power output that a solar system produces. This is measured as the number of kilowatt hours of solar power, per kilowatt of solar panels.

Using the infographic below, 1kW (1000W) of solar panels in Brisbane will produce 4.2kWh per day.

Solar output per kilowatt (1000 watts) installed, Australia

To convert kilowatts to watts, we divide the numbers above by 1000.

This means, 1000 watts of solar panels can produce around 4000 watt hours on average, in a day.

Another way to work out the average power from a solar panel is to use ‘Peak Sunlight Hours’. You can read about Peak Sun Hours in our post “What will a 100 watt solar panel run?‘.

If 12V solar panels typically come in multiples of 100, that 10 x 100 watt panels or 5 x 200 watt panels, and so on.

Combining 12v panels in strings like this can require expensive additional devices like combiners. It’s not the most efficient way to go about it.

Luckily for our example we don’t need 4000 watt hours of solar power.

We just need 1200 watt hours of solar power to charge our 100Ah battery. (Remember the conversion of Amps to Watts above?)

What size solar panel to charge a 100Ah battery?

To charge a 100AH, 1200 watt hour battery in 5 hours of sunshine you need a 240 watts of solar power.

1200W/5h = 240 watts.

You may think 300 watts of solar panels is sufficient, but that’s not always the case.

This is because solar panels don’t typically generate power at their nominal capacity. They don’t work at 100% efficiency. Solar panel power output can depend on the angle of the sun, shade lines and cloud cover, for example.

So it’s best to allow for this in your calculations by upsizing or oversizing your solar panels a little.

One option is to use 70% of the nominal capacity of the solar panel, for your calculations.

So 300 watts of solar panels will give you 210 watts of power output. Not enough power for the 240 watts you need to power your 100Ah battery.

400 watts of solar will give you 280 watts of power reliably and consistently.

So 400 watts of solar is more suitable to charge your 100Ah, 12 volt battery in all conditions.

2 x 200 watt 12 volt solar panels will cost around $600.

Does the way you use your battery matter?

You can’t fully discharge your battery each cycle. This will shorten the life of battery a lot!

So manufacturers provide a depth of discharge (DOD) recommendation for the battery. It’s usually a percentage of full discharge (the point where the battery is empty).

• Lithium ion batteries depth of discharge is optimised at around 80% (you can discharge the battery to 80% of its full discharge).
• Life P04 batteries depth of discharge is 90%+.
• Lead acid battery depth of discharge is only 50% (you can only half discharge it).

For the example above you won’t use the full 1200 watt hours, you’ll use around 960 watt hours instead.

Our calculations have used the full nominal capacity of the battery to be conservative. So you don’t run out of power. But you may get away with a 300 watt panel to charge a 100Ah 12V battery back to full in this case.

960 watts / 5 hours = 192 watts of solar power.

You just won’t have the same level of contingency built in if you do need to run the battery down more than 80%.

How quickly could I charge a 100Ah battery?

If you have more solar panels you could charge your 100Ah battery more quickly than 5 hours. 400 watts of solar panels could charge 1200 watt hour battery in 3 hours.

But you can’t upsize the solar panels indefinitely to charge the battery even faster.

This is because your battery will also have a maximum charge rate. It’s usually a number in watts. But it can also be a percentage of the battery’s nominal capacity.

For example the maximum charge rate for a lithium ion battery might be 30%. 30% of 1200 watts = 400 watts.

So you would be limited to 400 watts of solar panels when charging the battery.

400 watts of solar panels would charge the full 1200 watts in 3 to 4 hours, or 960 watts in 2 to 3 hours.

The manufacturer will generally specify the maximum charge rate. You can find it in the specifications or on the battery nameplate.

How do I keep the battery charged and run my appliances at the same time?

If you want to keep your battery charged and run your appliances at the same time, add the battery nominal capacity (1200 watts), to your total load.

Your total load is just your power usage.

Imagine you want to want to run a 12v water pump (uses 60 watts) plus an LED TV (that uses 50 watts). Your total load is 60 watts + 50 watts = 110 watts.

Add this to your 1200 watts of nominal battery capacity = 1300 watts.

1300 watts / 5 sunlight hours = 260 watts. You could use the same 400 watts of panels in this example.

How many solar panels will charge a 100Ah battery in 5 hours?

300 watts of solar panels is sufficient to charge a 100Ah battery in 5 hours. 400 watts of solar panels will do the same job more reliably, allowing for low sunlight due to clouding or the angle of the solar panels.

Conclusion

You can charge a 100Ah lithium ion battery in under 5 hours using either 300 or 400 watts of solar power. In this post, we’ve not only answered the question What size solar panel do I need to charge a 100Ah battery. We’ve also explained how to calculate it yourself. We recommend 400 watts of solar power if you can afford it. This will give you some extra capacity to use on cloudy days or in winter.

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